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Your problem is most likely related to MySQL. Less so HeidiSQL and least FreeBSD. You give precious little details but we can say the following:
Misconceptions:
The FreeBSD system "root" and the MySQL "root" users are not related!
You do not need to log into the FreeBSD system as "root" to work within the MySQL database as MySQL "root"
You do not need to ...

Promoting a comment to an answer: To quickly solve your problem, you should probably just try following the advice from our sister site, AskUbuntu, on how to reset your root (admin) password, which should cover you
Taking your question in detail, to explain what went wrong...
usr@usr-MS-7816:~$ su
Password:
su: Authentication failure
This would seem to ...